Range expansion (RE) also known as cell range expansion (CRE) is a useful feature in Long Term Evolution (LTE) heterogeneous network deployments. To meet expectations and predictions for high data rates and traffic capacity, the use of multiple low-output power sites to complement a macro cell has been developed, resulting in a heterogeneous network. The cells of the low-power nodes are sometimes referred to as micro or pico cells. Traditionally, a terminal connects to the node from which the downlink signal strength is the strongest. Due to differences in transmission power, this strategy does not necessarily result in that the terminal connects to the node with the lowest path loss, thereby selecting a node that does not provide the best uplink data rates.
The uptake area of a low-power node can be expanded without increasing the output power of the node by adjusting a cell selection offset to the received downlink signal strength during handling of the cell selection procedures. Such increase in the uptake area of a node is sometimes referred to as range expansion, range extension or cell selection offset. One purpose for doing RE may be to offload the macro cell by load sharing with the micro/pico node. If terminals are connecting to low-power nodes, the high-power node capacity is saved for other terminals to use. RE can therefore dynamically be applied to balance load between different layers (macro, micro and pico layers in a radio access network, RAN).
Another purpose for doing RE may be to improve the uplink (UL) performance for a User Equipment (UE) in a transition zone. The transition zone is the zone in which the downlink (DL) signal strength from the macro cell base station is higher than the DL signal strength from the micro/pico cell base station, but the path loss to the micro/pico cell base station is lower than to the macro cell base station. RE thereby improves the uplink received signal and link bitrate for the UEs that are subjects for the RE.
However, as a drawback RE degrades the DL. This is due to the fact that the UEs are connecting to the low power node, resulting in lower received signal strength and lower link bitrate in the DL, or more precisely bitrate per channel use. Moreover, for large RE the control signalling, such as synchronization signals, Cell-specific Reference Signals (CRS), from the macro cell can cause severe interference to UEs, connected to the micro/pico cell, within in the transition zone.
RE is typically applied by setting a cell selection offset parameter to a desired value. In the Third Generation Partnership Program (3GPP) standard “Radio Resource Control (RRC)”, technical specification (TS) 36.331, sections. 5.5.4.4 and 6.3.5., this parameter is called “cellIndividualOffset”.
When RE is applied, the typical approach is to modify the cell selection offset parameter for all UEs, which results in that all UEs in the transition zone will make a handover to the micro/pico node. This means that these UEs will experience more interference in DL from interfering nodes compared to what they experienced before the handover. Therefore, the total gain in radio resource utilization by applying RE is dependent on how capable the UEs are to mitigate interference. Thus, if there is a large fraction of UEs in the transition zone that are poor at interference suppression, then the decision of applying RE may become costly in terms of radio resources. In some scenarios a predicted gain may even result in a loss.
For UEs supporting release 10 of the 3GPP specifications, there will be some UEs having good interference suppression capability. Those UEs will likely belong to a new UE class, or category, of UEs supporting larger RE than other UEs. RE based on UE class can thus be performed by modifying the cell selection offset parameter only for UEs belonging to such a UE class. However, although two UEs belong to the same UE class there can still be large differences in their interference suppression capabilities.
A general problem with application of range expansion in prior art cellular communication systems is how to efficiently utilize the network and UE capabilities in order to have an effective mechanism for load sharing between different base station nodes in a RAN.